Abstract

This study was an attempt to combine two research literatures on hyperactivity: the
behaviour genetic research and the studies testing psychological theories of
hyperactivity. We obtained behavioural ratings from the teachers of 1316 twin
pairs, aged 7-12, from the general population. For a subsample of 268 twin pairs
we obtained ratings also from their parents. Forty-six hyperactive twin pairs (pairs
in which at least one twin was pervasively hyperactive) and 47 control twin pairs
were then assessed on tests relating to three theories of hyperactivity, those of
response inhibition deficit, working memory impairment and delay aversion.
Confirming previous findings, genetic factors accounted for 50-70% of the
variance in hyperactivity when considered as a continuous dimension. There was
also significant evidence of genetic effects on extreme hyperactivity, although the
present group heritability estimates were somewhat lower than previous estimates.
The hyperactive group performed worse than the control group on the delay
aversion measure and some of the working memory tasks. Controlling for IQ
removed the significant group differences on the working memory measures,
however. Although there were no significant group differences on the inhibition
variables, the inhibition measure, stop task, produced evidence of a pattern of
responding that was strongly characteristic of hyperactivity: hyperactive children
were variable in their speed, generally slow and inaccurate. This pattern of
responding may indicate a non-optimal effort/activation state.
To investigate the possibility that the cognitive impairments or task engagement
factors associated with hyperactivity mediate the genetic effects on the condition,
bivariate group heritability analyses were carried out. There was significant
evidence of shared genetic effects only on extreme hyperactivity and the variability
of speed. The findings are interpreted as supporting the state regulation theory of
hyperactivity. Although delay aversion is a characteristic of hyperactivity, it seems
to have an environmental rather than a genetic origin.